Mossberg 500 Tube Mount Picatinny Rail Holder

UPDATE: 3/13/2023 - Alternatively to suggestion from 3/3/2023, print with 4 layer walls, 50% infill, and gyroid infill pattern to increase longitudinal (direction of longest dimension) shear strength.

UPDATE: 3/8/2023 - I found that the option for the 3 inch rail section that comprises the top portion of the rail holder is weak at the second screw hole where the 3 inch rail attaches to the tube holder. It takes a pretty strong force, but the 3 inch rail section can be snapped at that point as it is the weak spot of the overhanging rail. One way to address this is by creating a series of through-holes (along the 3 inch dimension) at the base. I added Fusion 360 and STL files with the word "Improved" in the title. The through holes are a mere 0.01" in diameter, but when printed using thick walls they add structural support along the long axis. This helps strengthen the structure at the screw penetrations.

Designed specifically for the Mossberg 500. DO NOT ATTACH TO BARREL as the fixture will melt. This was designed to fit to the Mossberg 500 lower shell tube and does not touch the barrel when installed properly. Anti-rotation is provided by incorporating a small section of 1/16" rubber packing sheet (see link below). Rubber sheet section fits into recessed area on inner side of holder. Note this device is 1.5" long, so if you want this to fit just in front of the fore end on a 5-shot Mossberg500, you might need to reduce the length.

As always, I included the Fusion 360, STL, and a sliced file that has the settings below. Check out my picatinny rail design and flashlight holders as these were made to be compatible with this design (links below).

Recommend printing using a material such as PETG. Brand I use is in Print Setting section, below. I prototyped this device using both PLA and carbon fiber PETG and had a good fit with both. Final device version was printed in carbon fiber PETG.

Additional Materials:
1 - 1.5" long flat bottom picatinny rail section with 1" on center 4mm screw holes
(if you need to print one: https://www.thingiverse.com/thing:5880733)***
2 - 4 mm X 12 mm stainless button head hex screw
2 - 4 mm X 16 mm stainless button head hex screw
4 - 4 mm stainless nuts
loctite
1/16" rubber packing sheet cut to size (approximately) 1" width X 3.5" length

***Note that you might want an extended rail section if you have a Mossberg500 barrel shroud installed or if you have a 7 shot Mossberg500. You can go to the picatinny rail link above and modify the Fusion 360 file. Or you can use the STL or gcode file for the 3 inch rail section that I recently added.

Rubber packing sheet available at hardware stores such as:
https://www.lowes.com/search?searchTerm=rubber+packing+sheet

Installation:

1. Attach (use loctite) a 1.5" flat based picatinny rail section using 2 nuts and 2 - M4 X 16mm screws.
2. Pre-cut 1/16" using straight edge and exacto knife to dimensions above or larger.
3. Fit rubber sheet into middle of holder in 1" wide recessed area, mark and trim to exact dimensions such that sheet does not overlap. Make sure sheet is flush to walls of holder or the next step will be difficult.
4. Do I need to tell you to unload the shotgun?
5. Remove barrel from shotgun and slide holder with rubber sheet in place over tube. This may take some coaxing and you can use a coin to make the holder gap a bit wider to help.
6. Replace barrel.
7. Use remaining nuts and screws to secure rail at desired position. Once you loctite and tighten the screws, check that holder does not rotate.
8. Attach flashlight holder or other device to rail (my suggestion for an inexpensive weapon light holder: https://www.thingiverse.com/thing:5880792)